1. Field of the Invention
The present invention relates to a substrate for use of the ink jet recording head of an ink jet recording apparatus that forms droplets by discharging liquid from orifices. The invention also relates to a head using such substrate.
2. Related Background Art
With respect to an ink jet recording head of the kind, an ink jet recording method, such as disclosed in the specification of Japanese Patent Laid-open Application No. 54-51837, is to cause thermal energy to act upon liquid for obtaining the power source for discharging liquid. This is the characteristic aspect of the method that differs from the other types of ink jet recording methods. In other words, the recording method disclosed in the specification of the Laid-Open Application described above, liquid is heated by the application of thermal energy and is caused to bubble. The force generated by the bubbles as they expand, causes droplets of liquid to be ejected out through orifices arranged at the leading end of the recording head unit. These ejected droplets impinge upon and adhere to a recording member and thereby record information on the recording medium.
A recording head which operates according to the recording method described above is generally provided with several orifices through which the liquid is discharged. Such head also includes as part of its structure a liquid discharging unit which contains heat activating portions, which connects with the orifice. In the heat activating portions thermal energy acts upon the liquid therein and causes droplets to be ejected through the orifices as described above. In the heat activating portions there are provided heat generating resistive layers that form electrothermal transducing elements which generate thermal energy; upper layers are provided to protect the heat generating resistive layers from ink. In addition lower layers are provided under the heat generating resistive layers to accumulate heat.
Also, in the specification of Japanese Patent Laid-open Application No. 57-72867, it has been proposed to incorporate an element for driving heat generating resistors on a substrate in order to minimize the numbers of required electrodes pads.
FIG. 12 is a plan view which shows an example of a conventional recording head substrate structure having electric power wiring arranged on a substrate together with heat generating resistors.
The example shown in FIG. 12 is a substrate used for a so-called edge shooter type ink jet recording head where liquid is discharged in the direction substantially in parallel with the heat generating surface of heat generating resistors (in the right-hand direction in FIG. 12).
In the example of FIG. 12, a heat generating resistive layer and electrode layer are provided on a silicone substrate. Then, by means of photolithographic technique, heat generating elements 71 and pads 73 for use of external fetch electrodes are formed. The size of each heat generating resistor 71 is 150 pmxc3x9730 xcexcm. Eight resistors are provided at a pitch of 200 xcexcm.
Subsequently, a protection layer is formed over the resistive and electrode layer. Then by means of a photolithographic technique, a common electrode 72 and electrode pads 73 are formed over the protective layer. Through holes 74 are provided in the common electrode 72 by making holes on a fetching unit of common electrode. The common electrode 72 and its electrode pads 73 are formed from an aluminum layer which is subjected to a photolithographic and etching technique to shape a common electrode 72 and an electrode pad 75 extending therefrom which is used for external fetching.
In the conventional recording head thus structured, each of the electrode pads 73 is connected via through holes and associated electrodes with one end of each heat generating resistor 71, while the other end of each heat generating resistor is connected with the common electrode 72 by way of associated ones of the through holes 74 for its shareable use. Thus, heat is generated when voltage is applied across the electrodes 73 and 75.
Each of the heat generating elements 71 is separated and covered by the walls (not shown) which are arranged between and over them to form associated liquid flow paths. Liquid supplied into the flow paths is discharged from each of the orifices (not shown) by the force of expanding bubbles which are formed in the liquid by heat generated by the associated heat generating elements.
A large number of electrode pads are required to receive electric power which is supplied from an external source through each of the electrode pads. In order to maximize printing speed, a large number of heat generating resistors should be provided. It frequently happens that many of these several heat generating resistors must be driven simultaneously. When driving many heat generating resistors at the same time, a large amount of current must be supplied to the ink jet head.
The driving of an ink jet head which uses thermal energy to discharge ink by bubbling is different from the driving of a thermal head. To produce a good bubbling effect, the electrical current pulses which are applied to the heat generating resistors should be as short as possible. Accordingly, the electrical current in these pulses is increased. Thus, even if the electric power wiring which transmits these current pulses is arranged with a low resistance, there is still a problem encountered in that the quality of printed images becomes inferior. This is due to impediments, such as the inability to effectuate normal bubbling or disabled bubbling, because the voltage which causes each current pulse is caused to drop by an amount which corresponds to the product of the difference that takes place in the electric current when only one heat generating resistor is driven and when many of them are driven at the same time. Also the resistive value of the electric power wires further contributes to this voltage drop because this inevitably results in a reduction of the voltage applied to the heat generating resistors when several of them are driven at a time.
The above described problems can be appreciated from the following description in which the specific numerical values are given. When thirty-two heat generating resistors, each having a resistance of 1 xcexa9, are driven together and arranged with the driving current of 0.2 A for each with a heat generating resistor, the total current flow is 32xc3x970.2 or 6.4 A. When one of the resistors is driven along the total current flow is 1xc3x970.2 or 0.2 A; so that the difference in total current flow when one of them is driven and when all of them at the same time, is 6.2 A.
When the driving voltage is set at 20 V, which is 1.3 times the bubbling voltage 15.3 V, the driving voltage 13.8 V, which is 20 Vxe2x80x94such reduced voltage of 6.2 V, is lower than the bubbling voltage of 15.3 V. As a result, bubbling becomes impossible. In order to avoid this event, the applied voltage should be raised. However, if the applied voltage is raised, each of the heat generating resistors receives a greater voltage when each of them is driven individually. Therefore, the life of heat generating resistors is made shorter inevitably.
In convention practice each driving cycle is divided into several time increments and different groups of the heat generating resistors are driven in each time increment. Under the present circumstances, however, driving should be carried out at a high frequency in order to enhance the printing speed. Thus, the driving cycle should be as short as possible. However, the driving cycle duration is dependent primarily on the response capability of the driving element. It is difficult, because of the limited response capability of the driving element, to make the width of the driving pulse small and therefore only a limited number of driving pulses can be generated during each cycle. As a result, the number of time divisions cannot be increased any more.
Also, conceivably, it may be possible by varying the driving pulse components for the different voltage drops when different numbers of heat generating resistors are energized at the same time, by varying the width of the driving pulses so that wider pulses are produced when voltage drops become larger so that the amount of energy being used remains essentially constant. In this case, however, there is a need for the provision of a logic circuit that controls pulse width based on voltage drop to maintain a constant energy flow. This additional provision of a logic circuit leads to an inevitable increase in the manufacturing costs of the driving elements.
Also, it may be possible to make the wiring from a thick film by means of plating techniques or the like so that the resistance of the electric power wiring is made lower. In this case, however, a protection layer should be provided, because there is a possibility that the wires will otherwise come into contact with the ink. The provision of a protection layer on the thick film, however, causes its upper surface to be higher than the surface of the heat generating resistors. This, in turn, makes it difficult to form nozzle members downstream of the heat generating resistors, thus presenting another restriction in this respect. This is a particular problem because the recording head should be formed with very small ink flow channels so that ink droplets can be discharged with high precision. Specifically the nozzles should have a diameter in the order of 10 xcexcm, while the plated thick film wiring is also in the order of 10 xcexcm. In such case the problem is particularly conspicuous.
In order to reduce the resistance of the electric power wiring, it is generally necessary to make the electric power wires thicker. In such case the size of the substrate should be made larger accordingly. The manufacturing costs of the substrate increases significantly when a large number of heat generating elements are incorporated into the substrate. This is because the heat generating elements represent a large percentage of the cost of manufacturing the printing heads. In order to avoid this large cost, it may be conceivable to increase the number of pads which receive current from external fetch electrodes and thereby reduce the electrical resistance of an external wiring plate. However, increasing the number of pads not only reduces the reliability, it also requires the use of a larger substrate.
In order to solve the problems described above, the present invention provides in a substrate for an ink jet recording head, a plurality of heat generating resistors for discharging ink, as well as novel wiring arrangements for applying externally supplied electric power. According to these novel arrangements the heat generating resistors are divided into a plurality of groups with associated wiring. The wiring for each group has substantially the same wiring resistive value from its respective electrode pads arranged together therewith for receiving the supply of electric power from outside to its respective heat generating resistors.
In addition, a driving element may be incorporated within the substrate for driving heat generating resistors.
Further, the wiring for the plural groups may be connected together in the vicinity of the electrode pads.
Also, the electrode pads may be arranged on the edge portion of the substrate so that they are arranged in a direction different from the arrangement direction of the heat generating resistors.
According to another aspect, the present invention provides a novel method for driving a substrate structured as described above. This novel method is characterized in that time divisional driving is performed for the respective heat generating resistors.
According to a still further aspect the present invention provides a novel ink jet head which incorporates a substrate which is structured as described above.
According to yet another aspect of the invention there is provided a novel ink jet head cartridge which incorporates the ink jet head as described above.
In another aspect of the invention there is provided a novel liquid discharge apparatus having an ink jet head as described above, together with means for supplying driving signals in order to discharge liquid from such ink jet head.
In accordance with the present invention as described above, it is possible to arrange the resistive values of the wiring to be almost the same from the electrode pads provided together with the heat generating resistors to receive the supply of electric power from outside up to each of the heat generating resistors, thus making the amount of voltage drop smaller for each of the heat generating resistors when all of them are driven and when each of them is driven, respectively. Then, when time divisional driving is used to reduce the number of heat generating resistors being driven simultaneously, it becomes possible to reduce the number of divided groups within the substrate, thus producing more favorable effect. Particularly, it is preferable to perform separate driving for each block of the divided wiring.
Also, by incorporating the driving element on the substrate, it is possible to arrange the electric power wiring freely on the driving element. This facilitates both the division of the wiring and the adjustment of the wiring resistive values.
Here, in particular, the numbering of fetching connections can be reduced by dividing the electric power wiring into groups within the substrate and by connecting each group with associated electrode pads for external fetching.
Also, for an ink jet head which discharges ink vertically from the heat generating resistors, the present invention provides an advantage which is obtainable by arranging the pads for external fetching on the edge portions perpendicular to the arrangement direction of the heat generating resistors. In this way, the pad area can be made smaller. Also, it becomes easier to arrange each of the nozzle arrays.
In the cases described above, the electric power wiring can be divided for its effective arrangement to make the size of substrate smaller, leading to the significant reduction of costs of manufacture.